It has often been the case in recent years to use an aluminum alloy or an aluminum alloy composite material (MMC) for vehicles to reduce their weight. It is needless to say that a reduction in the weight of the brake system is also important and in an internally expanding type drum brake, for example, there has been sought an art using an aluminum alloy composite material (MMC) for the brake drum rotating with a wheel.
As regards the art using an aluminum alloy composite material (MMC), there is known an art adopting it for pulleys as disclosed in, for example, JP-A-2001-316740.
An art adopting an aluminum alloy composite material (MMC) for heat dissipating members is known from, for example, JP-A-2002-66724.
The process for manufacturing a pulley as disclosed in JP-A-2002-316740 will be described with reference to FIG. 10 hereof.
The pulley 301 shown in FIG. 10 has a hub 302 of a composite material in its center, an aluminum alloy disk 303 formed integrally with the hub 302 and a grooved portion 305 fitted about the disk 303 with a shock absorbing member 304 inserted therebetween, and a force for tightening bolts for securing the pulley 301 to a shaft is borne by the hub 302 of high strength.
Referring to the process for manufacturing the pulley 301, the hub 302 is first formed by extrusion molding a cylinder from a composite material and cutting it. Then, the hub 302 is set in a pulley casting mold and a molten aluminum alloy is poured into the mold.
The pulley manufacturing process described above requires the separate steps of making two parts, the hub 302 of a composite material and the disk 303 of an aluminum alloy, and its production calls for a great deal of time and labor. The working conditions in the manufacturing process of both the hub 302 and the aluminum alloy disk 303 are complicated and troublesome.
The process for manufacturing a composite material disclosed in JP-A-2002-66724 will now be described with reference to FIGS. 11A, 11B and 11C.
A product 311 of a composite material as shown in FIG. 11A includes a base portion 312 and a plurality of fins 313 formed on a surface of the base portion 312.
Firstly, a metal matrix composite material 314 is produced from an aluminum alloy 315 and fine particles 316 of silicon carbide and formed into a block 317, as shown in FIG. 11B. Secondly, the block 317 which has been heated is placed in a mold 318 (having fin spaces 319) and compressed.
When it is compressed, the aluminum alloy 315 flows into the fin spaces 319 and forms aluminum alloy fins 313, as shown in FIG. 11C.
According to this process for manufacturing a composite material, however, it is impossible to incorporate fine particles of silicon carbide adequately into the fins 313, though it may be possible to save the time and labor for its manufacture, and the fins 313 are exclusively of an aluminum alloy and too low in strength. It is impossible to distribute a desired proportion by volume of silicon carbide in the center of each fin 313 and as a result, it is difficult to utilize the strength of a composite material effectively.
Thus, there is sought an art which makes it possible to increase the strength of the disk portion of a brake drum of a metal matrix composite material (MMC) and manufacture a brake drum having a disk portion of improved strength without calling for a great deal of time and labor.